How many induvidual keys are used as hotkey shortcuts?
I couldn't really give you a number, but I do know they have a documented list of all the shortcuts they use. It should be listed in your user preferences (because you can remap them), but you can also find a list if you search for the version you're using. 

Please Take Notice: Many of the shortcuts changed for 2.8. If you're planning on using that version at any point in the future, consider taking a look at those shortcut keys, instead.